Designing the foam liner: the cheapest place to protect an expensive instrument
Material, density, layering and drawing handoff — the liner decisions that decide whether the payload arrives.
Short answer
- A liner carries load into the case walls and stops the payload accelerating inside the shell.
- EPE (polyethylene bead foam) is cheap, tough and has open-ish cells that can crumble at edges; good for base layers.
- Match density to the payload’s mass and the drop height you care about.
- Send a drawing with the payload outline in the three orientations, pocket depth, wall thickness, and the tolerance you will accept.
The liner is a structure, not a packing
A liner carries load into the case walls and stops the payload accelerating inside the shell. That means pocket walls must transfer force, not just cradle shape. Design it as three zones: a base that absorbs the drop, side support that stops lateral travel, and a top restraint for the orientation the case will spend most of its life in.
Material choice by consequence
EPE (polyethylene bead foam) is cheap, tough and has open-ish cells that can crumble at edges; good for base layers. EVA is denser, cleaner-cut and presents better, at a higher cost — the usual choice for the visible top layer. PU (polyurethane) pluck foam is the no-tooling option for prototypes and mixed payloads. Cross-linked PE is the fine-cell option when appearance and tear matter.
Density and thickness
Match density to the payload’s mass and the drop height you care about. A heavy instrument with a thin, soft liner bottoms out; a light instrument in hard dense foam transmits more shock, because there is nothing to compress. When in doubt, go thicker and softer in the base, firmer in the side restraint.
Handoff to the factory
Send a drawing with the payload outline in the three orientations, pocket depth, wall thickness, and the tolerance you will accept. DXF works for CNC knife cutting; a physical payload plus a case sample works better when pockets are complex. State the anti-static requirement as a surface resistivity number, not as "anti-static", or you will get ordinary foam with a pink colourant.
The two most common field failures
Pockets cut to the payload’s cosmetic outline with no clearance for a connector or a cable, and layered liners bonded with an adhesive that fails in heat. Both are free to fix at drawing stage and expensive to fix after a shipment.
Questions buyers ask
Should the liner be one block or layered?
Layered. It is cheaper to produce, allows different densities where the shock enters, and lets the end user reconfigure the pockets by swapping a top layer.